1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus. More particularly, the present invention is concerned with in image forming apparatus of the type forming a thin layer of dense and viscous developing liquid on a developer carrier and causing it to contact the surface of an image carrier in order to develop a latent image formed thereon by electrophotography, electrostatic recording or ion flow scheme, and a wet developing method for the same.
2. Discussion of the Background
An image forming apparatus with a wet developing device for developing a latent image electrostatically formed on a photoconductive element or similar image carrier is conventional. To reduce the overall size of the image forming apparatus, the size of the wet developing device may be reduced by using, e.g., a dense and viscous developing liquid consisting of an insulating liquid and toner dispersed therein and having a high toner content. Japanese Patent Laid-Open Publication 7-209922, for example, teaches a method of developing a latent image formed on an image carrier with charged toner particles. In accordance with this method, a developing liquid consisting of an insulating liquid and toner dispersed therein and having a viscosity as high as 100 mPa.s to 10,000 mPa.s is applied to a conductive developer carrier and then brought into contact with the image carrier. As a result, the developing liquid is fed to the latent image formed on the image carrier.
The above document also teaches that a prewetting liquid is applied to the image carrier before the development of the latent image. The presetting liquid is an insulating liquid having a parting ability and chemically inactive. The prewetting liquid forms a layer separating the layer of developing liquid fed to the image carrier and the surface of the image carrier. This prevents needless toner from depositing on the surface of the image carrier and thereby protects an image from disturbance ascribable to toner deposition on a non-image area.
The document further teaches that at least one of the image carrier or photoconductive element and developer carrier is formed of an elastic material so as to scatter a contact pressure to act between the developer layer formed on the developer carrier and the photoconductive element. With this configuration, when the developer layer formed on the developer carrier and the photoconductive element contact each other, the developer layer is prevented from being crushed and disturbing the image.
The document additionally teaches that the developer layer formed on the developer carrier has its thickness physically reduced by a series of contact rollers. This kind of thinning method is similar to a method of feeding a thin film of ink to a master in, e.g., an offset printer.
However, the physical scheme for thinning the dense and viscous developing liquid on the developer carrier needs several to several tens of rollers, resulting in a complicated structure and difficult maintenance. Specifically, if the number of rollers is reduced in order to simplify the roller arrangement, then the developer forms ribs on the developer carrier due to the high viscosity of the developer. This makes it difficult to form a thin uniform developer layer on the developer carrier. Consequently, irregularity occurs in the amount of developer to be fed to the photoconductive element and prevents an image from being uniformly developed. The resulting image has an irregular density distribution.
By contrast, an offset printer forming a thin film of ink by a method similar to the above method is free from the irregular image density distribution. This is presumably because the developing liquid for use in the wet developing device contains charged toner particles and therefore differs in electrical characteristic from the ink of the offset printer.